Laser welding is a metal joining process in which a laser beam is directed at a metal workpiece stack-up to provide a concentrated energy source capable of effectuating a weld joint between the overlapping constituent metal workpieces. In general, two or more metal workpieces are first aligned and stacked relative to one another such that their faying surfaces overlap and confront to establish a faying interface (or faying interfaces) that extends through an intended weld site. A laser beam is then directed towards and impinges a top surface of the workpiece stack-up. The heat generated from the absorption of energy from the laser beam initiates melting of the metal workpieces down through the metal workpiece impinged by the laser beam and into the underlying metal workpiece(s) to a depth that intersects each of the established faying interfaces. And, if the power density of the laser beam is high enough, a keyhole is produced within the workpiece stack-up. A keyhole is a column of vaporized metal, which may include plasma, derived from the metal workpieces. The keyhole is surrounded by molten workpiece metal and is an effective absorber of energy from the laser beam, thus allowing for deep and narrow penetration of molten workpiece metal into the stack-up compared to instances in which a keyhole is not present.
The laser beam melts the metal workpieces in the workpiece stack-up in very short order once it impinges the top surface of the stack-up. After the metal workpieces are initially melted, a beam spot of the laser beam may be moved across the top surface of the workpiece stack-up along a predefined path. As the beam spot of the laser beam is advanced along the top surface of the stack-up, molten workpiece metal flows around and behind the advancing beam spot. This penetrating molten workpiece metal quickly cools and solidifies into resolidified composite metal workpiece material. Eventually, the transmission of the laser beam at the top surface of the workpiece stack-up is ceased, at which time the keyhole collapses and any molten workpiece metal still remaining within the stack-up solidifies. The collective resolidified composite metal workpiece material obtained by directing the laser beam at the top surface of the stack-up and advancing the beam spot of the laser beam along a weld path constitutes a laser weld joint and serves to autogenously fusion weld the overlapping metal workpieces together.
The automotive industry is interested in using laser welding to manufacture parts that can be installed on a vehicle. In one example, a vehicle door body may be fabricated from an inner door panel and an outer door panel that are joined together by a plurality of laser weld joints. The inner and outer door panels are first stacked relative to each other and secured in place by clamps. A laser beam is then sequentially directed at multiple weld sites around the stacked panels in accordance with a programmed sequence to form the plurality of laser weld joints. The process of laser welding inner and outer door panels—as well as other vehicle component parts such as those used to fabricate hoods, deck lids, body structures such as body sides and cross-members, load-bearing structural members, engine compartments, etc.—is typically an automated process that can be carried out quickly and efficiently. The aforementioned desire to laser weld metal workpieces together is not unique to the automotive industry; indeed, it extends to other industries that may utilize laser welding including the aviation, maritime, railway, and building construction industries, among others.
The use of laser welding to join together coated metal workpieces that are often used in manufacturing practices can present challenges. For example, steel workpieces often include a zinc-based surface coating (e.g., zinc or a zinc-iron alloy) for corrosion protection. Zinc has a boiling point of about 906° C., while the melting point of the underlying steel substrate it coats is typically greater than 1300° C. Thus, when a steel workpiece that includes a zinc-based surface coating is laser welded, high-pressure zinc vapors are readily produced at the surfaces of the steel workpiece and have a tendency to disrupt the laser welding process. In particular, the zinc vapors produced at the faying interface(s) of the steel workpieces can diffuse into the molten steel created by the laser beam unless an alternative escape outlet is provided through the workpiece stack-up. When an adequate escape outlet is not provided, zinc vapors may remain trapped in the molten steel as it cools and solidifies, which may lead to defects in the resulting laser weld joint—such as porosity—as well as other weld joint discrepancies including blowholes, spatter, and undercut joints. These weld joint deficiencies, if sever enough, can unsatisfactorily degrade the mechanical properties of the laser weld joint.
Steel workpieces that are used in manufacturing practices may also include other types of surface coatings for performance-related reasons in lieu of zinc-based coatings. Other notable surface coatings include aluminum-based coatings such as aluminum, an aluminum-silicon alloy, an aluminum-zinc alloy, or an aluminum-magnesium alloy, to name but a few examples. Unlike zinc-based surface coatings, aluminum-based surface coatings do not boil at a temperature below the melting point of steel, so they are unlikely to produce high-pressure vapors at the faying interface(s) of the workpiece stack-up. Notwithstanding this fact, these surface coatings can be melted, especially if a keyhole is present, and, when in a molten state, can combine with the molten steel derived from the bulk of the steel workpieces. The introduction of such disparate molten materials into the molten steel can lead to a variety of weld defects that have the potential to degrade the mechanical properties of the laser weld joint. Molten aluminum or aluminum alloys (e.g., AlSi, AlZn, or AlMg alloys), for instance, can diminish the purity of the molten steel and form brittle Fe—Al intermetallic phases within the weld joint as well as negatively affect the cooling behavior of the molten steel.
Aluminum workpieces are another intriguing candidate for many automobile component parts and structures due to their high strength-to-weight ratios and their ability to improve the fuel economy of the vehicle. Aluminum workpieces, however, almost always include a surface coating that covers an underlying bulk aluminum substrate. This coating may be a refractory oxide coating that forms passively when fresh aluminum is exposed to atmospheric air or some other oxygen-containing medium. In other instances, the surface coating may be a metallic coating comprised of zinc or tin, or it may be a metal oxide conversion coating comprised of oxides of titanium, zirconium, chromium, or silicon, as disclosed in U.S. Patent Application No. US2014/0360986, the entire contents of which are incorporated herein by reference. The surface coating inhibits corrosion of the underlying aluminum substrate through any of a variety of mechanisms depending on the composition of the coating and may provide other favorable enhancements as well.
One of the main challenges involved in laser welding aluminum workpieces is the relatively high solubility of hydrogen in molten aluminum. Upon solidification of the molten aluminum, dissolved hydrogen becomes trapped, leading to porosity within the laser weld joint. In addition to the challenges posed by hydrogen solubility, the surface coating commonly included in the aluminum workpieces is believed to contribute to the formation of weld defects in the laser weld joint. When, for example, the surface coating of one or more of the aluminum workpieces is a refractory oxide coating, residual oxides can contaminate the molten aluminum created by the laser beam due to the high melting point and mechanical toughness of the coating. In another example, if the surface coating is zinc, the coating may readily vaporize to produce high-pressure zinc vapors that may diffuse into and through the molten aluminum, thus leasing to porosity within the weld joint and other weld deficiencies unless provisions are made to vent the zinc vapors away from the weld site. A variety of other challenges may also complicate the laser welding process in a way that adversely affects the mechanical properties of the laser weld joint.
Magnesium workpieces are yet another intriguing candidate for many automobile component parts and structures due to their high strength-to-weight ratios—even more so that aluminum workpieces—and their ability to improve the fuel economy of the vehicle. Like aluminum workpieces, magnesium workpieces almost always include a surface coating that covers an underlying bulk magnesium substrate. This coating may be a refractory oxide coating that forms passively when fresh magnesium is exposed to atmospheric air or some other oxygen-containing medium. In other instances, the surface coating may be a metallic conversion coating comprised of metal oxides, metal phosphates, or metal chromates. The surface coating included in the magnesium workpiece can help protect the underlying magnesium substrate against protection through any of a number of mechanisms and may also contribute to other favorable properties as well.
The laser welding of magnesium workpieces has historically been more challenging when compared to steel and aluminum workpieces. The major challenge involved in laser welding magnesium workpieces is porosity in the laser weld joint. Such porosity may be derived from entrapped gas in the micropores of the bulk magnesium substrates of the magnesium workpieces, which can undergo expansion and coalescence in the molten magnesium, especially when the magnesium workpieces include a die cast magnesium alloy substrate. Weld joint porosity can also be derived from other factors including the rejection of dissolved hydrogen during solidification of the molten magnesium created by the laser beam. Still further, when the surface coating of the magnesium workpiece(s) is a refractory oxide coating, the magnesium hydroxide component (due to exposure to humidity) of the surface coating can evolve water vapor when heated by the laser beam. The evolved water vapor may diffuse into and through the molten magnesium and contribute to entrained porosity within the laser weld joint. A host of other challenges may also may also disturb the laser welding process and contribute to the formation of a laser weld joint with degraded mechanical properties.